1. Field of the Invention
The present invention relates to a motor apparatus, manufacturing method, exposure apparatus, and device fabrication method.
2. Description of the Related Art
A projection exposure apparatus has conventionally been used when manufacturing a fine semiconductor device such as a semiconductor memory or logic circuit by using the photolithography technique. In the projection exposure apparatus, a projection optical system projects patterns (circuit patterns) formed on a reticle (mask) onto a substrate such as a wafer, thereby transferring the circuit patterns. Note that the projection exposure apparatus includes a reticle stage and wafer stage for respectively sequentially moving the reticle and wafer relative to the projection optical system by using a linear pulse motor or dual-axis pulse motor when projecting the reticle patterns onto the wafer.
An outline (the arrangement and operation principle) of the linear pulse motor will be explained below with reference to FIGS. 13A to 13D and 14.
In FIGS. 13A to 13D, reference numeral 1010 denotes a platen that is a stator of the linear pulse motor, and has periodical convex portions (platen convex portions) 1012 and concave portions (platen concave portions) 1014 on the surface. Note that the platen convex portions 1012 are arranged at a predetermined interval τ on the platen 1010. Reference numeral 1020 denotes a movable portion having movable-portion teeth (convex portions) 1022 on the surface. Reference numeral 1030 denotes a coil for magnetizing the movable-portion teeth 1022 by a current. Reference numeral 1040 denotes a permanent magnet that generates attraction between the platen 1010 and movable-portion teeth 1022. Reference numerals 1051, 1052, 1053, and 1054 respectively denote first, second, third, and fourth salient poles that generate magnetic attraction by the coils 1030. Reference symbol MF denotes a magnetic flux generated by the currents flowing through the coils 1030 and the permanent magnet 1040.
FIG. 13A shows a state in which the movable portion 1020 is in an original position. FIG. 13B shows a state in which the movable portion 1020 is in a τ/4 position. FIG. 13C shows a state in which the movable portion 1020 is in a 2τ/4 position. FIG. 13D shows a state in which the movable portion 1020 is in a 3τ/4 position. The first to fourth salient poles 1051 to 1054 of the movable portion 1020 are spatially shifted by τ/4.
First, when a current is supplied in a direction shown in FIG. 13A to the coil 1030 corresponding to the first salient pole 1051 while the movable portion 1020 is in the original position (FIG. 13A), a magnetic flux passing through the first salient pole 1051 combines with the magnetic flux of the permanent magnet 1040 and becomes maximum. Consequently, the first salient pole 1051 and the platen convex portions 1012 immediately below the first salient pole 1051 attract each other. When a current is supplied in a direction shown in FIG. 13A to the coil 1030 corresponding to the second salient pole 1052, a magnetic flux passing through the second salient pole 1052 cancels the magnetic flux of the permanent magnet 1040. Consequently, the second salient pole 1052 is positioned above the platen concave portions 1014. Also, each of the third and fourth salient poles 1053 and 1054 and the platen convex portions 1012 obliquely attract each other.
Then, the currents supplied to the coils 1030 corresponding to the first and second salient poles 1051 and 1052 are stopped. When currents are supplied in directions shown in FIG. 13B to the coils 1030 corresponding to the third and fourth salient poles 1053 and 1054, a magnetic flux passing through the fourth salient pole 1054 combines with the magnetic flux of the permanent magnet 1040 and becomes maximum. As a result, the fourth salient pole 1054 and the platen convex portions 1012 immediately below the fourth salient pole 1054 attract each other. A magnetic flux passing through the third salient pole 1053 is canceled by the magnetic flux of the permanent magnet 1040, so the third salient pole 1053 and platen convex portions 1012 do not attract each other any longer. Also, each of the first and second salient poles 1051 and 1052 and the platen convex portions 1012 obliquely attract each other.
Thus, the movable portion 1020 moves to the τ/2 position shown in FIG. 13B so as to oppose the fourth salient pole 1054 to the platen convex portions 1012. Similarly, the movable portion 1020 can sequentially be moved to the right by switching the directions of the currents to be supplied to the coils 1030 as shown in FIGS. 13A to 13D.
In the linear pulse motor, a predetermined clearance is generally held between the platen 1010 and movable portion 1020 by an LM guide or an air bearing formed in the movable portion 1020. As shown in FIG. 14, therefore, a non-magnetic epoxy resin ER is filled in the platen concave portions 1014 and between the movable-portion teeth 1022, thereby forming an air-bearing running surface. This air-bearing running surface is required to have high surface accuracy. In the manufacturing process of the platen 1010, therefore, after the epoxy resin ER is filled in the platen concave portions 1014 and cured, the surface (facing the movable portion) of the platen 1010 is ground to secure predetermined surface accuracy. Note that this surface grinding is performed by wet grinding in order to prevent expansion of the epoxy resin ER caused by grinding heat generated by friction between a whetstone of surface grinding and the surface of the platen 1010, thereby preventing deterioration of the surface accuracy. Note also that after the surface of the platen 1010 is ground, the platen convex portions 1012 are normally plated or coated as a rust preventing process.
Next, an outline of the dual-axis pulse motor will be explained below with reference to FIGS. 15 to 17.
FIG. 15 is a view showing the entire dual-axis pulse motor. A movable portion 2020 floats by about 20 μm on a platen 2010 having platen convex portions 2012, and can move in the X and Y directions. More specifically, as shown in FIG. 16, the movable portion 2020 has air discharge holes 2060 for floating it by discharging air against the platen 2010, and is formed by arranging two uniaxial movable portions 1020 shown in FIGS. 13A to 13D in each of the X and Y directions. As shown in FIG. 17, the platen 2010 has the periodical platen convex portions 2012 (in the form of a matrix) and a platen concave portion 2014 on the surface. The platen concave portion 2014 is filled with the non-magnetic epoxy resin ER.
The surface (opposite to the movable portion) of the platen 2010 is an air-bearing running surface of the movable portion 2020, and hence required to have high surface accuracy. In the manufacturing process of the platen 2010, therefore, after the epoxy resin ER is filled in the platen concave portion 2014 and cured, the surface of the platen 2010 is ground by wet surface grinding and subjected to a rust preventing process (plating or coating), in the same manner as for the platen 1010.
Generally, the thrust of a motor apparatus such as the linear pulse motor or dual-axis pulse motor as described above is proportional to the square of the magnetic flux density in the gap (magnetic gap) between the movable portion and platen (stator). Note that the magnetic gap is specifically a gap from the platen convex portions to the movable-portion teeth.
The magnetic gap is equal to the floating amount of the air bearing when no rust preventing process is performed on the platen, and equal to the sum of the thickness of a plating layer or coating layer and the floating amount of the air bearing when the rust preventing process is performed on the platen. As the magnetic gap decreases, the magnetic flux density increases, so the thrust of the motor apparatus increases.
Although the sensitivity of the thrust of the motor apparatus to the magnetic gap depends on the configuration of a magnetic circuit, this sensitivity is generally about 0.5 to 2%/μm. The floating amount of the air bearing is about 5 to 10 μm, and the critical value of the thickness of the plating layer is about 50 μm. If the floating amount of the air bearing is 5 μm or less, the movable portion and platen (stator) come in contact with each other because, for example, the movable portion deforms. Also, if the thickness of the plating layer is 50 μm or less, the plating layer peels off because the adhesion is insufficient. When the rust preventing process is performed, therefore, even a minimum magnetic gap must be 55 to 60 μm.
These techniques are proposed in Japanese Patent Laid-Open No. 2006-141080.
Unfortunately, the above-mentioned motor apparatuses have the following four problems.
The first problem is the decrease in thrust of the motor apparatus caused by the rust preventing process. When a plating layer is formed on the platen convex portions as the rust preventing process, the thrust of the motor apparatus decreases in proportion to the thickness of the plating layer as described previously. This decrease in thrust of the motor apparatus can be suppressed by minimizing the thickness of the plating layer. However, if the thickness of the plating layer is smaller than 50 μm, the plating layer peels off because the adhesion is insufficient. Accordingly, it is very difficult to decrease the magnetic gap to 55 μm or less.
The second problem is the fluctuation in thrust of the motor apparatus caused by the difference between the platen convex portion and the surface of the epoxy resin filled in the platen concave portion. In the conventional motor apparatus as described above, the air-bearing running surface is formed by filling the platen concave portion with the epoxy resin, and ground by wet surface grinding. In this wet surface grinding, a grinding solution swells the epoxy resin. Since this produces a difference of 5 to 10 μm between the platen convex portion and the surface of the epoxy resin, the floating amount of the air bearing changes within the range of the pitch of the movable-portion teeth. As a consequence, the magnetic gap between the movable portion and platen (stator) changes, and the thrust of the motor apparatus fluctuates.
The third problem is the decrease in thrust of the motor apparatus caused by deterioration of the surface accuracy of the air-bearing running surface. As described above, the platen concave portion is filled with the epoxy resin. Volume shrinkage when this epoxy resin cures deforms the whole platen, and this deteriorates the surface accuracy of the air-bearing running surface. In particular, the peripheral portions of the platen readily deform (warp). As the movable portion moves to the peripheral portion of the platen, therefore, the magnetic gap widens, and this decreases the thrust of the motor apparatus.
The fourth problem is the complexity of the manufacturing process caused by the rust preventing process. When forming a plating layer of the surface of the platen, no plating layer adheres to the epoxy resin (surface) filled in the platen concave portion. This produces a difference equal to the thickness of the plating layer between the platen convex portion and the surface of the epoxy resin. To eliminate this difference, it is necessary to coat the platen surface with the epoxy resin again, and perform finishing surface grinding. Thus, the rust preventing (plating layer formation) step, epoxy resin filling step, and surface grinding step must be repeated several times to manufacture the platen. This complicates the manufacturing process and increases the manufacturing cost.